Giant Breach in Earth's Magnetic Field Discovered

Dec.
16, 2008: NASA's five THEMIS spacecraft have discovered
a breach in Earth's magnetic field ten times larger than anything
previously thought to exist. Solar wind can flow in through
the opening to "load up" the magnetosphere for powerful
geomagnetic storms. But the breach itself is not the biggest
surprise. Researchers are even more amazed at the strange
and unexpected way it forms, overturning long-held ideas of
space physics.

The
magnetosphere is a bubble of magnetism that surrounds Earth
and protects us from solar wind. Exploring the bubble is a
key goal of the THEMIS mission, launched in February 2007.
The big discovery came on June 3, 2007, when the five probes
serendipitously flew through the breach just as it was opening.
Onboard sensors recorded a torrent of solar wind particles
streaming into the magnetosphere, signaling an event of unexpected
size and importance.

Right:
One of the THEMIS probes exploring the space around Earth,
an artist's concept. [more]

"The
opening was huge—four times wider than Earth itself,"
says Wenhui Li, a space physicist at the University of New
Hampshire who has been analyzing the data. Li's colleague
Jimmy Raeder, also of New Hampshire, says "1027
particles per second were flowing into the magnetosphere—that's
a 1 followed by 27 zeros. This kind of influx is an order
of magnitude greater than what we thought was possible."

The
event began with little warning when a gentle gust of solar
wind delivered a bundle of magnetic fields from the Sun to
Earth. Like an octopus wrapping its tentacles around a big
clam, solar magnetic fields draped themselves around the magnetosphere
and cracked it open. The cracking was accomplished by means
of a process called "magnetic reconnection." High
above Earth's poles, solar and terrestrial magnetic fields
linked up (reconnected) to form conduits for solar wind. Conduits
over the Arctic and Antarctic quickly expanded; within minutes
they overlapped over Earth's equator to create the biggest
magnetic breach ever recorded by Earth-orbiting spacecraft.

Above:
A computer model of solar wind flowing around Earth's magnetic
field on June 3, 2007. Background colors represent solar wind
density; red is high density, blue is low. Solid black lines
trace the outer boundaries of Earth's magnetic field. Note
the layer of relatively dense material beneath the tips of
the white arrows; that is solar wind entering Earth's magnetic
field through the breach. Credit: Jimmy Raeder/UNH. [larger
image]

The
size of the breach took researchers by surprise. "We've
seen things like this before," says Raeder, "but
never on such a large scale. The entire day-side of the magnetosphere
was open to the solar wind."

The
circumstances were even more surprising. Space physicists
have long believed that holes in Earth's magnetosphere open
only in response to solar magnetic fields that point south.
The great breach of June 2007, however, opened in response
to a solar magnetic field that pointed north.

"To
the lay person, this may sound like a quibble, but to a space
physicist, it is almost seismic," says Sibeck. "When
I tell my colleagues, most react with skepticism, as if I'm
trying to convince them that the sun rises in the west."

Here
is why they can't believe their ears: The solar wind presses
against Earth's magnetosphere almost directly above the equator
where our planet's magnetic field points north. Suppose a
bundle of solar magnetism comes along, and it points north,
too. The two fields should reinforce one another, strengthening
Earth's magnetic defenses and slamming the door shut on the
solar wind. In the language of space physics, a north-pointing
solar magnetic field is called a "northern IMF"
and it is synonymous with shields up!

"So,
you can imagine our surprise when a northern IMF came along
and shields went down instead," says Sibeck.
"This completely overturns our understanding of things."

Northern
IMF events don't actually trigger geomagnetic storms, notes
Raeder, but they do set the stage for storms by loading the
magnetosphere with plasma. A loaded magnetosphere is primed
for auroras, power outages, and other disturbances that can
result when, say, a CME (coronal mass ejection) hits.

The years ahead
could be especially lively. Raeder explains: "We're entering
Solar Cycle 24. For reasons not fully understood, CMEs in
even-numbered solar cycles (like 24) tend to hit Earth with
a leading edge that is magnetized north. Such a CME should
open a breach and load the magnetosphere with plasma just
before the storm gets underway. It's the perfect sequence
for a really big event."

Sibeck
agrees. "This could result in stronger geomagnetic storms
than we have seen in many years."

What
happened to conventional wisdom? Researchers
at the University of New Hampshire are using computer
models to unravel the basic physics of the great breach.
They're finding that reconnection at the poles is key.
Conventional wisdom held that equatorial reconnection
was more important, which is why the giant breaches
were not anticipated until THEMIS flew through one.

THEMIS,
short for Time
History of Events and Macroscale Interactions during
Substorms, is the fifth medium-class mission under
NASA's Explorer Program. The program, managed by The
Explorers Program Office at Goddard Space Flight Center,
Greenbelt, Md., provides frequent flight opportunities
for world-class space investigations in Heliophysics
and Astrophysics. The University of California, Berkeley's
Space Sciences Laboratory managed the project development
and is currently operating the THEMIS mission. Swales
Aerospace, Beltsville, Md., built the THEMIS satellites.